Simplify C++ coroutine usage

[snarkmaster]

Use more standard coroutine constructs to make the code shorter & easier to follow.

I'm not a coro expert, but maybe some of this was necessary with older compilers. However, looking at RedHat:

• Fedora 36 is the earliest release that has kernel 6.0
• The same Fedora has gcc 12.2.1, which compiles this simpler code just fine

Test Plan:

make
./tests/run_test.sh all 10 tests/tmp/

[snarkmaster]

Ok, I brought back the macro, but named it co_await__suspend_always(tag) so that it's somewhat clear what it does. Let me know what you think.